How do I get one hair coupon?

Depends on which one you are looking for. Originally they were found in Fringe Magazines (totally random which coupon you received) There are some in the Cash Shop and you can find others in trades or the shop search. HERE is a list of the hair coupons on Subeta, and what Fringe magazine they came from if you are wanting to risk the random chance (though for cheaper ones its better to buy them rather than use a fringe magazine IMHO)

Other ways to get coupons besides what mentioned is by buying them through other users. You could try the Shop Search, or, try looking in the Advertising forums or by posting in the Wanted forum. Users often buy and sell items there, often for a discount, and you can work out good deals there. If you're looking for a specific coupon this is a great option.